This invention relates generally to combustion chambers and, more particularly, to means for effectively cooling the liners thereof. Although the present invention will be described in terms of a combustion chamber for use in gas turbine engines, it will be understood that the structure as contemplated is suitable for any high temperature combustion apparatus requiring film convection cooling.
Increased efficiency in gas turbine engines is accomplished, in part, by an increase of the operating temperature in the combustor. However, in order to withstand these high temperatures with an acceptable life term, it is necessary not only to employ highly sophisticated alloys and materials, but to provide an efficient and reliable means for cooling the liners of the combustion chambers.
One of the most efficient techniques for cooling the combustor liner is that of film convection cooling wherein a protective film boundary of cool air is made to flow along the inner surface of a liner so as to insulate the liner from the adjacent hot gases of combustion. The cooling air film not only forms a protective barrier between the liner and the hot gases, but also provides for convective cooling of the liner.
Introduction of the cooling air into the combustion liner is generally accomplished by way of a plurality of circumferentially spaced holes which provide fluid communication from a surrounding cooling air plenum to a plurality of axially spaced annular lipped pockets in the inner side of the liner. As cooling air enters the holes, it tends to mix and coalesce within the pocket. The air is then directed by the lip to flow rearwardly so as to attach to and flow along the inner surface of the liner.
It will be recognized that in order for the lip to provide the required directing function to the flow of air, it is necessarily cantilevered rearwardly a substantial distance so as to define with the outer liner surface, a slot for controlling the discharge of the thin film of cooling air. In order to prevent this slot from partial closing by the thermal outward growth of the lip, it has become common practice to provide small dimples or props in circumferentially spaced relationship around the lip to prevent the buckling tendency induced by the thermal stresses. While the inclusion of dimples in this manner serves well to overcome lip distortion, the dimples have been found to create wakes in the film of cooling air discharged along the inner surface of the liner. The wakes tend to destroy the uniformity of the cooling air barrier and permit hot gases of combustion to directly contact the inner liner of the combustor to thereby reduce its operating life.
U.S. Pat. Nos. 3,826,082 and 4,050,241, issued on July 30, 1974, and Sept. 27, 1977, and assigned to the assignee of the present invention, describe specific dimple construction for the elimination of the problems associated with the use of dimples, as described hereinabove. Although the solutions as proposed have, to a great degree, been successful, the dimples or props are still exposed to very hot temperatures and resultant high stresses which lead to short life of the dimples or props themselves. Further, even though the dimples are designed so as not to disrupt the flow of cooling air through the slot, they still tend to provide some restriction with resultant local wakes and hot streaking.
A combustor liner design which has to some extent overcome the difficulties as described hereinabove, is that shown in U.S. Pat. No. 3,978,662, issued on Sept. 7, 1976, and assigned to the assignee of the present invention. One feature of that design was a modified lip design which, because of its shorter length, tends to be less susceptible to thermal buckling. However, it should be recognized that the lip is still located in the hot gas stream and is subject to both high thermal stresses and thermal buckling, which would tend to close the gap and thus create disruptions in the cooling airflow.
It is, therefore, an object of the present invention to provide a combustor liner design with improved performance characteristics.
Another object of the present invention is the provision in a combustor liner film cooling slot for the prevention of a partial closing of the slot by thermal growth of the associated lip.
Yet another object of the present invention is the provision in a combustor liner cooling slot for the substantial elimination of hot streaking downstream thereof.
Still another object of the present invention is the provision in a liner cooling slot for a plurality of props which are not susceptible to high stresses and thus limited life resulting from exposure to high temperature gases.
Yet another object of the present invention is the provision for a combustor cooling liner which is effective in use and economical to manufacture.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.